As the dimensions of semiconductor devices decrease, it is becoming increasingly difficult and expensive to fabricate nanoscale features and precisely control dopants at the nanoscale level. A great deal of research has been devoted to extending the capabilities of UV lithograph due to its high throughput for manufacturing as compared to direct-write processes such as electron beam lithography. Modern UV lithography machines are rapidly approaching the limits of the smallest features possible, and alternative methods are required to achieve reproducible nanoscale features and doping profiles. Retrograde doping in MOS-FET devices to reduce short channel effects and graded base doping in bipolar junction transistors for reduction of base transit times are two examples of the importance of nanoscale impurity doping.
One method to achieve nanoscale structures and precision of homogeneous implants is to use high aspect ratio structures etched into resist at an angle to the substrate to serve as implantation masks. The high aspect ratio of these masks reduces lateral ion straggle and helps keep the implanted profiles sharp and well-defined. For gradient doping, thermal diffusion is typically used to grade the profile which, in practice, is difficult to precisely control at the nanoscale level.
The growing number of applications for superconductor quantum interference devices (SQUIDs) such as quantum computing and low noise amplifiers is also driving the need for more economical fabrication processes. To provide an example, the use of step-edge junctions are known to provide increased device yield and enable fabrication of complex circuits including microbridge junctions. Step-edge junctions are generally patterned using standard lithography followed by Argon ion milling, which is performed at an angle relative to the substrate plane to achieve the desired step angle. Depending on the substrate material, the formation of angled step edges is non-trivial due to the erosion of the mask material during the milling process, resulting in shallow step-edge profiles. As a result, standard lithography processes are frequently inadequate. One approach to solving this problem is described in U.S. Patent Publication No. 2004/0023434 of Venkatesan et al., which involves the use of a shadow mask with an overhang structure during ion milling. The overhang is formed using photoresist that is treated by intermixing curing steps with chlorobenzene treatments.
Reactive ion etching (RIE) is an established and widely used technique to selectively remove material from a surface. RIE is typically used in conjunction with a lithographically-patterned resist spun onto the sample surface in order to transfer the pattern into the substrate. A large number of variables consisting of types of gases used and plasma power enable RIE to be adapted to a wide range of applications in science as well as in the semiconductor industry.
The incident ions accelerated in the plasma are typically directed perpendicular to the sample surface, which, while providing for the formation of features with vertical sidewalls, makes any attempt to perform RIE at a non-perpendicular angle to the surface difficult. A few papers have been published describing the use of more or less complicated sample holders or faraday cages with the goal of placing the substrate inside the plasma dark space. As the ballistic transport of ions in the dark space ensures a high directionality, a tilted surface located inside this dark space will continue to be etched perpendicular to the cathode (typically the RIE sample holder), resulting in inclined sidewalls. Some publications report efforts to perform angled etching of substrates, however, these approaches typically include complicated sample holders that are limited to either small samples, or fixed angles. In addition, the described processes may not be used to create high aspect-ratio implant masks. Examples of publications on this subject include: B. Jacobs, R. Zengerle, “Reactive ion etching of sloped sidewalls for surface emitting structures using a shadow mask technique” (1996), J. Vac. Sci. Technol. B, 14(4), 2537-2542; R. W. Tjerkstra, L. A. Woldering, et al, “Method to pattern etch masks in two inclined planes for three-dimensional nano- and microfabrication” (2011), J. Vac. Sci. Technol. B, 29(6), 061604; T. Takamori, L. A. Coldren, J. L. Merz, “Angled etching of GaAs/AlGaAs by conventional Cl2 reactive Ion Etching”, (1988), Appl. Phys. Lett. 53 (25), 2549-2551.
The need remains for a method for creating masks that can be used to produce nanoscale features, such as doping profiles with arbitrary angles, MEMS, SQUIDs and other devices using conventional lithography processes. The present invention is directed to such a need.